Metallizations for components operating with acoustic waves, from which, by way of example, busbars, electrode fingers connected to the busbars or reflector structures are formed on a piezoelectric substrate, should preferably be designed to have particularly high power durability.
Bulk acoustic waves (BAW) or surface acoustic waves (SAW) propagate within a solid or at the surface thereof; their frequencies are in the GHz range.
The current-carrying (electrode) structures, which convert electrical radiofrequency signals into acoustic waves or conversely acoustic waves into electrical radiofrequency signals, therefore have to withstand firstly the electric currents and secondly the mechanical deformation as a result of the acoustic waves, without incurring damage.
U.S. Pat. No. 7,605,524 B2 discloses electrode structures for components operating with surface acoustic waves (SAW). In one embodiment, an electrode structure is arranged on monocrystalline lithium tantalate or lithium niobate. The electrode structure comprises a first layer composed of titanium. A second layer deposited thereon comprises aluminum. The second layer comprises two <1 1 1> domains. Methods for producing the electrode layers concern the epitaxial growth of the layer comprising aluminum. A highly textured layer comprising aluminum has good, i.e., high, electrical conductivity values and a high power durability.
The growth of a layer deposited on a substrate is called epitaxial if the alignment of the atoms of the layer is oriented to the alignment of the atoms of the substrate. One general problem in the case of epitaxial growth consists in providing a suitable surface. This is because the constitution of the surface of the substrate—as interface between substrate and layer—crucially influences the order of the atoms of the deposited layer. Therefore, substrates are usually pretreated by heat treatment or by incipient etching. Between the pretreatment and the actual deposition process, the surface of the substrate must no longer be contaminated with impurities.
The use of the lift-off technique when depositing electrode structures onto substrates is problematic if a particularly “clean” substrate surface is of importance. This is because, in the first step, a resist layer is applied to the possibly pretreated substrate surface. In further method steps, said resist layer is then partly exposed and the exposed regions are removed in a further process step. At the formerly exposed locations, the substrate surface is now uncovered again. However, this surface in the meantime has had contact both with the resist layer and with the solvent that removed the exposed resist layer. The substrate surface is therefore contaminated.
Epitaxial growth of electrode layers by means of the lift-off technique appeared hitherto to be possible only with great difficulty.
Another method, the so-called etching method, is based on the electrode material being applied to the possibly pretreated substrate surface over a large area and the electrode structures being obtained by etching away the undesirable covering regions of the electrode material: superfluous electrode material is removed.